1. Field of the Invention
The present invention relates to the field of fabricating photomasks for use in the manufacture of semiconductor devices and, more particularly, to the fabrication of binary optical photomasks for use in lithography.
2. Related Application
This application is related to copending application entitled "Attenuated Phase Shifting Mask With Buried Absorbers," Ser. No. 08/342,939, filed Nov. 21, 1994.
3. Prior Art
Conventional binary intensity masks generally contain patterned absorber films (such as chromium, chromium oxide or molybdenum) on a highly polished quartz substrate. These absorber regions on the photomask absorb light so as to totally absorb or significantly attenuate light passing through the region. Thus, dark and light image patterns are projected from the mask onto a target, such as a semiconductor wafer.
The absorber regions are formed on a surface of the quartz so as to present topographical features on the surface. The absorber features can have "rough" edges, as well as other topographical defects, such as cracking or peeling, which can contribute to light scattering at the vertical edges of the absorber. The interface of quartz, absorber material and air form a "triple" optical singularity point due to the intersection of three different indices of refraction.
Additionally, pellicles are used on prior art masks to protect the surface of the mask from being contaminated. This covering is necessary since the image plane is at the surface of the quartz. The attachment of a pellicle in a defect free manner is difficult and involves an expensive manufacturing process. Often, pellicles degrade under exposure to ultraviolet radiation, thereby limiting the useful life of the mask.
Because of these shortcomings, conventional binary optical masks have disadvantages affecting the cost of manufacture and useful life. However, these disadvantages are minimal compared to the "blurring" of the image experienced due to edge diffraction effects and the imperfect edge definition associated with the absorber regions. Without a sharp transition from dark to light, clear distinctive features are difficult to obtain and such distortions are amplified as device features shrink in size. At submicron levels, these edge distortions severely impact image contrast and resolution.
The present invention describes a photomask which addresses the disadvantageous qualities of the conventional binary intensity mask noted above by providing for a photomask having buried absorbers with a built-in offset that significantly reduces edge diffraction effects and eliminates the need for a pellicle.